Preserve blank lines within multiline values

[charliermarsh]

Summary

Given an INI file like:

[externally-managed]
Error=To install Python packages system-wide, try brew install
 xyz, where xyz is the package you are trying to
 install.

 If you wish to install a non-brew-packaged Python package,
 create a virtual environment using python3 -m venv path/to/venv.
 Then use path/to/venv/bin/python and path/to/venv/bin/pip.

 If you wish to install a non-brew packaged Python application,
 it may be easiest to use pipx install xyz, which will manage a
 virtual environment for you. Make sure you have pipx installed.

Python's configparser preserves the empty lines within the multiline value, while the configparser crate drops them.

This PR adds support for preserving those blank lines based on following the Python configparser rules, so...

• Empty lines after the value are dropped (even if there's another value that follows it).
• Empty lines that consist of just comments are dropped (even if they're indented).

[QEDK]

Good catch btw @charliermarsh also love the great work with Ruff. 🖖

[charliermarsh]

Thanks so much @QEDK! Glad I could find a moment to contribute here.